Cycloaddition aromatic compounds

In this section, reactivity studies will be emphasized while in those devoted to synthesis (Section 4.04.3) theoretical calculations on reactions leading to the formation of pyrazoles (mainly 1,3-dipolar cycloadditions) will be discussed. It should be emphasized that the theoretical treatment of reactivity is a very complicated problem and for this reason, most of the calculations have been carried out on aromatic compounds, as they are the easiest to handle. In general, solvents are not taken into account thus, at the best, the situation described theoretically corresponds to reactions taking place in the gas phase. 

The photochemical cycloadditions of alkenes and alkynes with aromatic compounds have received by far the most attention. Yields of [2+2] cydoadducts can be good, but reaction times are often long and secondary rearrangement products are common [139, 140, 141,142, 143,144, 145,146] (equations 63-65). The pioneering mechanistic and synthetic work on aromatic photocycloadditions has been reviewed [147],... 

Several examples of [5C+1S] cycloaddition reactions have been described involving in all cases a 1,3,5-metalahexatriene carbene complex as the C5-syn-thon and a CO or an isocyanide as the Cl-synthon. Thus,Merlic et al. described the photochemically driven benzannulation of dienylcarbene complexes to produce ortho alkoxyphenol derivatives when the reaction is performed under an atmosphere of CO, or ortho alkoxyanilines when the reaction is thermally performed in the presence of an isonitrile [111] (Scheme 63). In related works, Barluenga et al. carried out analogous reactions under thermal conditions [36a, c, 47a]. Interestingly, the dienylcarbene complexes are obtained in a first step by a [2+2] or a [3S+2C] process (see Sects. 2.3 and 2.5.1). Further reaction of these complexes with CO or an isonitrile leads to highly functionalised aromatic compounds (Scheme 63). 

Divinylnaphthalenes 63 and 64 (Figure 2.8) react with strong dienophiles and have been used to synthesize complex polycyclic aromatic compounds [60], While 2-vinylnaphthalene (61) and 1-vinylnaphthalene (62) act as 47t components, 3-vinylphenanthrene (65) and 1- vinylpyrene (66) (Figure 2.8), characterized by an extended polycyclic aromatic system, preferentially undergo [2- -2] cycloaddition [57, 61, 62]. 

Dihydro-1-vinylnaphthalene (67) as well as 3,4-dihydro-2-vinylnaphtha-lene (68) are more reactive than the corresponding aromatic dienes. Therefore they may also undergo cycloaddition reactions with low reactive dienophiles, thus showing a wider range of applications in organic synthesis. The cycloadditions of dienes 67 and 68 and of the 6-methoxy-2,4-dihydro-1-vinylnaphthalene 69 have been used extensively in the synthesis of steroids, heterocyclic compounds and polycyclic aromatic compounds. Some of the reactions of dienes 67-69 are summarized in Schemes 2.24, 2.25 and 2.26. In order to synthesize indeno[c]phenanthrenones, the cycloaddition of diene 67 with 3-bromoindan-l-one, which is a precursor of inden-l-one, was studied. Bromoindanone was prepared by treating commercially available indanone with NBS [64]. 

Dihydrovinylphenanthrenes are more reactive than the corresponding vinyl phenanthrenes and undergo Diels-Alder reactions easily. They have been used in the synthesis of polycyclic aromatic compounds and helicenes. Examples of cycloaddition reactions of the 3,4-dihydro-1-vinylphenanthrene (70), [61] 3,4-dihydro-2-vinylphenanthrene (71) [68] and l,2-dihydro-4-vinylphenanthrene (72) [69] are reported in Equation 2.22 and Schemes 2.27 and 2.28. 

Since dihydroarylethenes are more reactive than the corresponding fully aromatic compounds, their use in the cycloaddition reactions is preferred in order to carry out the reactions under mild conditions with higher yields. Some reactions of 3,4-dihydro-1-vinylnaphthalene (103) [33], 3,4-dihydro-2-vinyl-naphthalene (104) [34], and l,2-dihydro-4-vinylphenanthrene (105) [35] with 4-acetoxy-2-cyclopentenone (98) and 2-inden-l-one (106) are summarized in Schemes 5.11-5.13. 

Flamers R. J., Coulter S. K., Ellison M. D., Flovis J. S., Padowitz D. F., Schwartz M. P., Greenlief C. M., Russell J. N. Jr Cycloaddition Chemistry of Organic Molecules With Semiconductor Surfaces Acc. Chem. Res. 2000 33 617 624 Keywords carbonyi group, semiconductor materiais, surface reaction, aikenes, aromatic compounds, azo compounds, cycioaikadienes, isothiocyanates, unsaturated compounds... 

Elimination of nitrogen from D-A adducts of certain heteroaromatic rings has been useful in syntheses of substituted aromatic compounds.315 Pyrazines, triazines, and tetrazines react with electron-rich dienophiles in inverse electron demand cycloadditions. The adducts then aromatize with loss of nitrogen and a dienophile substituent.316... 

When benzyne is generated in the absence of another reactive molecule it dimerizes to biphenylene.132 In the presence of dienes, benzyne is a very reactive dienophile and [4+2] cycloaddition products are formed. The adducts with furans can be converted to polycyclic aromatic compounds by elimination of water. Similarly, cyclopentadienones can give a new aromatic ring by loss of carbon monoxide. Pyrones give adducts that can aromatize by loss of C02, as illustrated by Entry 7 in Scheme 11.9. 

Another example of the preparing of aromatic compounds via the Diels-Alder reaction of nitroalkenes is presented in Eq. 8.16.26 Cycloaddition of methyl propiolate affords a high yield of the isomeric product. 

In recent years, the importance of aliphatic nitro compounds has greatly increased, due to the discovery of new selective transformations. These topics are discussed in the following chapters Stereoselective Henry reaction (chapter 3.3), Asymmetric Micheal additions (chapter 4.4), use of nitroalkenes as heterodienes in tandem [4+2]/[3+2] cycloadditions (chapter 8) and radical denitration (chapter 7.2). These reactions discovered in recent years constitute important tools in organic synthesis. They are discussed in more detail than the conventional reactions such as the Nef reaction, reduction to amines, synthesis of nitro sugars, alkylation and acylation (chapter 5). Concerning aromatic nitro chemistry, the preparation of substituted aromatic compounds via the SNAr reaction and nucleophilic aromatic substitution of hydrogen (VNS) are discussed (chapter 9). Preparation of heterocycles such as indoles, are covered (chapter 10). 

H(65)1889, 2005EJO3553>. Starting dihydro[l,2,4]triazolo[3, 4-4]benzo[l,2,4]triazines 482 readily react with aromatic aldehydes to yield iminium salts 483. These salts treated with a base (e.g., triethylamine) are deprotonated to reactive 1,3-dipolar azomethine imines 484. In contrast to related five-membered heterocycles, these compounds are relatively unstable on storage in the solid form and particularly in solution. Fortunately, this obstacle can be easily circumvented by their in situ preparation and subsequent 1,3-dipolar cycloaddition. These compounds can participate in 1,3-dipolar cycloadditions with both symmetric and nonsymmetric dipolarophiles to give the expected 1,3-cycloadducts in stereoselective manner. Selected examples are given in Scheme 82. 

A Dotz benzannulation reaction was utilized in the synthesis of the furo[2,3- >]furan core of aflatoxin B2 as illustrated below <06TL2299>. Synthesis of polynuclear aromatic compounds was achieved by using [5+5] cycloaddition of 2-alkynylarylcarbene complexes and enyne-aldehyde derivatives <06TL5303>. 

Moreno described the cycloaddition of 2,5-dimethylfuran (42) catalyzed by silica-supported Lewis acids under solvent-free conditions in closed Teflon vessels using a commercial microwave oven (Scheme 9.11) [28, 52]. Under these conditions coordination of the silica-supported catalyst with the oxygen bridge favors ring opening, thus leading to the aromatic compounds in one step. The use of Si (71) gave the best results for aromatic compounds. 

Cycloaddition with nitrile oxides occur with compounds of practically any type with a C=C bond alkenes and cycloalkenes, their functional derivatives, dienes and trienes with isolated, conjugated or cumulated double bonds, some aromatic compounds, unsaturated and aromatic heterocycles, and fullerenes. The content of this subsection is classified according to the mentioned types of dipolarophiles. Problems of relative reactivities of dienophiles and dipoles, regio- and stereoselectivity of nitrile oxide cycloadditions were considered in detail by Jaeger and... 

The cydoaddition of different 1,3-dipoles such as azides [331, 341] and diazoalkanes [342-344] to acceptor-substituted allenes was thoroughly investigated early and has been summarized in a comprehensive review by Broggini and Zecchi [345], The primary products of the 1,3-dipolar cycloadditions often undergo subsequent fast rearrangements, for example tautomerism to yield aromatic compounds. For instance, the five-membered heterocycles 359, generated regioselectively from allenes 357 and diazoalkanes 358, isomerize to the pyrazoles 360 (Scheme 7.50) [331]. 

Intermolecular [4+2]-cycloaddition of vinylallenes with alkynes is efficiently mediated by means of an electronically tuned rhodium catalyst (Scheme 16.81) [91]. A five-membered rhodacycle is formed from the vinylallene. Coordination followed by insertion of an alkyne to the rhodacycle generates a seven-membered rhodacycle, from which rhodium(I) is eliminated reductively to produce a cyclohexatriene, leading to the aromatic compound. 

Somewhat unusual examples, which illustrate the usefulness of cycloadditions in the synthesis of polycyclic compounds, are the controlled synthesis of an unsymmetrically substituted aromatic compound (131 from very simple commercially available compounds (Scheme 6.9), and the one-step thermal cyclisation of compound J6 to the aromatic steroid 14 (Scheme 6.10). 

Fullerenes, among which the representative and most abundant is the 4 symmetrical Cgg with 30 double bonds and 60 single bonds, are known to behave as electron-deficient polyenes rather than aromatic compounds [7]. The energy level of the triply degenerate LUMO of Cgg is almost as low as those of p-benzoquinone or tetracyanoethylene. Thus, a wide variety of reactions have been reported for Cgg such as nucleophilic addition, [4-1-2] cycloaddition, 1,3-dipolar addition, radical and carbene additions, metal complexation, and so on [7]. Fullerene Cgg also undergoes supramolecular complexation with various host molecules having electron-donating ability and an adequate cavity size [8]. 

Spirocychc C-ribosyl aromatic compounds have also been derived from o-ribono-1,4-lactone by a similar strategy. In this case a mild cylopentadienyl mthenium complex was used as catalyst for the cycloaddition step [114]. 

When 2D or 3D PAHs are considered then the choice becomes much larger. An exhaustive review on the advances in the synthesis of polycyclic aromatic compounds can be found in Harvey, 2004. The most used strategies are flash vacuum pyrolysis, cross-coupling, oxidative photocyclization, Diels-Adler cycloaddition, etc. 

Thus by several criteria l,4-dihydro-l,4-diazocines without a strongly electron-withdrawing group at the 1,4-positions are aromatic compounds, whereas those with such groups are non-planar systems with localized bonding. Diene character in the latter compounds is seen in the cycloaddition of N- phenyltriazolinedione to give the adducts (234b and c). 

Photocycloadditions of naphthalene derivatives to alkcnes have been recently reviewed.60 Examples of such reactions are the photocycloaddition of naphthalene to 2,3-dihy-drofuran,61 of 4-methoxy-l-naphthonitrile to acrylonitrile62 and of 2-trimethylsiloxynaph-thalene to methyl acrylate.63 2-Naphthols undergo cycloaddition with ethene in the presence of aluminum trihalides only.64 Other bicyclic aromatic compounds, e.g. A-acylindoles65-67 and /V-methylphenanthrene-9,10-dicarboximide,68 have also been studied in detail. Irradiation of 5/f-dibenzo[u,i7]cyclohepten-5-one (21) and dimethyl 2-methylfumarate (22) in dioxane gives the cyclobutane adduct 23 in 73% yield.69... 

The major classes of photochemical reaction for aromatic compounds are nucleophilic substitution and a range of processes that lead to non-aromatic products—valence isomerization, addition or cycloaddition reactions, and cyclization involving 6-electron systems. These five general categories of reaction will be described in the following sections, together with a few examples of more specific processes. 

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